Diffusion of mobile products produced in reactive barrier membranes was studied theoretically and experimentally. A diaphragm cell in which a reactive solute challenges a reactive membrane was adopted for this purpose. A diffusion equation with a reaction term describes the behavior of a mobile product before breakthrough of the solute, while the common diffusion equation governs the system after the breakthrough. An approximate, analytical solution was derived to this diffusion problem based on a pseudo-steady state assumption, and the result agreed closely with a numerical solution without assumptions. A poly(vinyl alcohol) membrane containing zinc oxide particles was challenged by hydrochloric acid in a diffusion experiment to validate the theoretical predictions. Diffusion of zinc chloride, the mobile reaction product, was monitored using a colorimetric method. The experimental results matched the analytical model. (c) 2007 Elsevier B.V. All rights reserved.